1. Introduction
This invention relates to tungsten carbide tools and to the incorporation of Group IVB and VB metals into said tools.
2. Description of the Prior Art Most tungsten carbide tools are manufactured by the well known powder metalurgical technique of blending tungsten carbide powders with cobalt powders followed by pressing and sintering. The cobalt acts as a binder for the tungsten carbide powders. In some cases, especially to prevent crater formation, it is known in the art to blend titanium carbide and tantulum carbide powders with the tungsten carbide.
In recent years, it has been suggested that improved tool life of tungsten carbide tools containing titanium carbide was caused by the formation of titanium oxide layers, and in some cases sulfide layers and other oxides, on the surface of the tool. This belief was based upon the showing by electron microscopy that a protective and adherent layer of Ti.sub.2 O.sub.3 was present as an intermediate layer between a TiC-TiO solid solution on the surface of the carbide tool after cutting.
It has been more recently shown that by application of cutting fluids, sulfide and oxide layers are formed in the contact zone of a carbide cutting tool and an aluminum-deoxidized steel, in which case the sulfide and oxide layers are less than 10 .mu.m thick. It has been asserted in the prior art that the oxide layer formed on the surface of the tungsten carbide tool raises the welding temperature between the carbide tool and steel by several hundred degrees, thereby decreasing wear by the "adhesion-and-shear" process. In addition, it has also been asserted that TiO is very adherent to the carbide surface of the tool since the sub-surface forms a solid solution of TiO and TiC. Thus, in the prior art, it was believed that the role of the surface layer was that of reducing the frictional coefficients and preventing diffusion and the like between the tool and the workpiece.
With regard to the above described oxide layers, though they are identifiable, the stability of these layers under the shear stresses encountered during normal cutting operations was not considered until recently. Thus, it is conceivable that the oxide layer may not be able to withstand the high shearing stresses encountered in metal cutting for a prolonged period of time and that the rate of oxide formation at the metal to metal contact may not be rapid enough to provide a continuous oxide layer. It was then considered possible that what really effected the metal cutting property was not necessarily the oxide layer alone, but the substrate layer of the carbide effected by mass diffusion of the oxide layer into the carbide tool.
In accordance with the above, the prior art then deliberately attempted to diffuse a titanium oxide layer into a carbide tool by applying a paste of titanium oxide over the surface of the tool and heating the tool to an elevated temperature to thereby diffuse the titanium oxide into the surface. Substantially improved results were obtained.
In addition to the above, it is well known in the art to provide tungsten carbide tools having a layer of another carbide such as titanium carbide displaced over its surface to improve properties such as wear life. These layers are typically formed by gaseous decomposition of the desired metal of the carbide in a carbon atmosphere such as methane. Such tools are disclosed in U.S. Pat. No. 3,604,689. It has been found that where the coating is firmly adhered to the surface of the tool, the wear life is improved and typically is about two times that of an untreated tool. However, difficulty has been experienced with said tools in that there is a tendency for the carbide coating to flake off the tool thereby decreasing its useful life.
With regard to said carbide coated tungsten carbide tools, it has been considered undesirable to diffuse said carbide coating into the body of the tool and such diffusion is prevented or does not take place. This is discussed by McCrary, "T.C. Coatings", Proceedings of the 2nd Carbide Cutting Tool Seminar, Purdue University, June 17-18, 1971.